This invention relates to an adaptive equalizer subsystem employed in fixed broadband wireless access (FBWA) applications operating in adaptive short-burst modems and multi-link hopping mesh radio networks over slow time-varying channels. The adaptive modem is capable of fast link-hopping from one link to another over such channels. That is, the channel is quasi-static from burst to burst for any given link.
The embodiments described herein may be used in conjunction with a wireless mesh topology network of the type described in U.S. patent application Ser. No. 09/187,665, entitled xe2x80x9cBroadband Wireless Mesh Topology Networksxe2x80x9d and filed Nov. 5, 1998 in the names of J. Berger and I. Aaronson, with carrier phase recovery system described in U.S. patent application Ser. No. 09/764,202, entitled xe2x80x9cCarrier recovery System for Adaptive Modems and Link Hopping Radio Networksxe2x80x9d and filed in the names of M. Rafie et al., and with network nodes including switched multi-beam antenna designs similar to the design described in U.S. patent application Ser. No. 09/433,542, entitled xe2x80x9cSpatially Switched Router for Wireless Data Packetsxe2x80x9d and filed in the names of J. Berger, et al., as well as with the method and apparatus disclosed in U.S. patent application Ser. No. 09/699,582 entitled xe2x80x9cJoin Process Method For Admitting A Node To A Wireless Mesh Network, filed Oct. 30, 2000 in the names of Y. Kagan, et al. Each of these U.S. patent applications is incorporated in its entirety herein by reference. Other applications for the embodiments will be apparent from the description herein.
Burst transmission of digital data is employed in several applications such as satellite time-division multiple access, digital cellular radio, wide-band mobile systems and broadband wireless access systems. The design trade-offs and the resulting architectures are different in each of these applications.
In general, the receiver must filter the received burst waveforms in a way that will result in the best possible bit-error performance. In most cases, this means maximizing the ratio of signal power to power of noise, interference, and distortion. In modern systems, this implies using a matched filter or an adaptive equalizer.
In most of these applications, a preamble of known symbols is inserted in the beginning, middle, or at end of each burst of data packets for training purposes. Such an approach is not appropriate in applications involving transmission of short bursts. The insertion of a known data sequence greatly reduces the transmission efficiency for a short burst. As a result, preamble-based algorithms are not applicable in such systems.
Ideally, it is highly desirable to minimize the use of training sequences for initial acquisition or subsequent adaptation. This property is especially important for short-burst formats used in many existing wireless communication applications that utilize Time-Division Multiple Access (TDMA) such as IS-136, GSM, EDGE, and fixed broadband wireless access systems. Short burst formats are used to reduce end-to-end transmission delay and to limit the time variation of wireless channels over a burst. However, training overhead can be very significant for such short burst formats. This overhead ranges up to 30% in many systems. The overhead of these systems can be recovered by employing the adaptive equalization apparatus outlined in this invention. In cases where longer range or higher tolerance of delay spread is needed, adaptive equalization can be used for these systems without changing physical link formats.
A constant need for ever-increasing throughputs through fixed bandwidths, fueled by broadband Internet protocol (IP) applications, has pushed system designers toward more throughput-efficient modulation schemes. Because of their relatively good performance, large quadrature amplitude modulation (QAM) constellations are being used in many of these applications. One of the critical problems associated with the use of large QAM constellations is that of amplitude and delay distortion of the radio link, which for efficiency reasons, must often be done without the use of a preamble, particularly in burst modem systems.
There are several classes of approaches to adaptive equalization. Low complexity algorithms for adaptation, such as the least mean-square (LMS) error algorithms, are fairly common in adaptive equalizers. Faster approaches, such as the least-squares (LS), recursive least-squares (RLS), fast Kalman, and square-root RLS methods, require computationally-intensive matrix inversions and (in some cases) stability issues. Adaptive equalizers can be further classified into linear transversal and recursive structures.
In transversal (tap-delay-line) equalizers, the current and past values of the received symbols, r(t-nT), are linearly weighted by equalizer tap coefficients (tap gains) c(n) and summed to produce the equalized signal,       y    ⁢          (      n      )        =            ∑      k        ⁢          xe2x80x83        ⁢                  c        ⁢                  (          k          )                    ⁢                        r          ⁢                      (                                          t                0                            +              nT              -              kT                        )                          .            
A zero-forcing (ZF) equalizer minimizes the peak distortion of the worst case ISI (inter-symbol interference) only if the peak distortion before equalization is less than 100 percent. In an LMS equalizer, however, the equalizer tap coefficients are chosen to minimize the mean-square errorxe2x80x94the sum of squares of all the ISI (inter-symbol interference) terms plus the noise power at the output of the equalizer.
Under the class of non-linear receiver structure, various optimality criteria related to error probability are considered. This culminated in the development of the maximum likelihood sequence estimator (MLSE) using the Viterbi algorithm (VA) and adaptive version of such a receiver. The computational complexity of the MLSE is proportional to mLxe2x88x921, which grows exponentially with symbol alphabet size (m) and the number of terms in the discrete channel pulse response (L). Another branch of non-linear and sub-optimal receiver structure is the decision-feedback equalizer (DFE). A decision-feedback equalizer makes memoryless decisions and cancels all trailing inter-symbol interference (ISI) terms. DFE, however, suffers from a reduced effective signal-to-noise ratio (SNR) and error propagation, due to its inability to defer decisions.
Fast convergence is important for adaptive equalizers in receivers polling multi-point networks where each node in the network must adapt to receive typically short bursts of data from a number of transmitters over different radio links. Orthogonalized LMS algorithms are used to speed up equalizer convergence. In particular, a self-orthogonalization technique, such as RLS and adaptive lattice (AL) are used for rapidly tracking adaptive equalizers. Kalman (RLS) and fast Kalman algorithms obtain their fast convergence by orthogonalizing the adjustment made to the coefficients of an ordinary linear transversal equalizer. Adaptive lattice algorithms, on the other hand, use lattice filter structure to orthogonalize a set of received signal components. In some applications, use of fast converging equalizers are avoided due to computational complexity and stability issues.
If the impairments that the equalizer must resolve are small enough so that the modem can successfully track timing and carrier phase prior to equalization, then the equalizer can be made to train much more rapidly. For more severely distorted channels, an approach that trains the equalizer prior to recovery of timing and carrier may be needed.
The effect of carrier phase error, xcfx86e=xcfx86xe2x88x92{circumflex over (xcfx86)}, in high-level modulation schemes, such as M-QAM is to reduce the power of the desired signal component by a factor of cos2(xcfx86xe2x88x92{circumflex over (xcfx86)}) in addition to the cross-talk interference from the in-phase and quadrature components. Since the average power level of the in-phase and quadrature components is the same, a small phase error causes a large degradation in performance of the adaptive equalizer, particularly at higher modulation levels (i.e., Mxe2x89xa716). An accurate carrier phase recovery unit described in U.S. patent application Ser. No. 9/764,202, entitled xe2x80x9cCarrier recovery System for Adaptive Modems and Link Hopping Radio Networksxe2x80x9d and filed on Jan. 17, 2001 in the names of M. Rafie, et al., is used following a non-adaptive pre-equalizer and before two adaptive equalizers in order to reduce the adverse impact of the carrier phase offset on the performance and the convergence of the adaptive equalizer.
In continuous modem applications, the user is typically willing to wait a time period while the receiver goes through an acquisition phase in which tracking processes adapt the tap coefficients of the equalizer. Often, the tap-convergence process in a continuous modem simply allows the adaptive equalizer to keep on tracking the channel impulse response and the undesired noise (interference plus noise) continuously based on the received signal. In other words, the acquisition processing is not different from the tracking processing.
In contrast, in a burst modem, the user data content of a given transmission may be only a fraction of a millisecond. Long acquisition times contribute an unacceptable level of overhead to the system and substantially reduce capacity. Thus, the burst modem requires a special acquisition process that will quickly estimate the appropriate receiver gain, the carrier frequency and phase, the sample timing frequency and phase, and the tap coefficients for an equalizer of the receiver. Also, the acquisition process must reliably identify which bit in the burst is the first user data bit so that higher layers of the protocol stack can format data properly.
The initial tap coefficient values can be estimated using a training sequence in the join (acquisition) mode of the system. A QPSK signaling sequence may be used in the join mode to compensate the amplitude and delay distortion of each individual radio link. The tap coefficient values may then be used as the initial tap weights for the fixed pre-equalizer and the iterative adaptive equalizer as will be described in the sequel
Conventional equalization in wireless communications requires frequent transmission of training sequences. This represents a system overhead and effectively reduces the information rate. On the other hand blind equalization techniques do not require training sequences. One of the most popular blind algorithms is the family of constant modulus algorithm (CMA). There are several disadvantages in using the CMA family of algorithms. One of them is the existence of local minima. Another drawback of blind algorithm is the slow convergence and inability to achieve equalization in a short burst.
Hence, there is a need for a method and apparatus for an adaptive equalizer technique in a burst-mode system. Further, there is a need for a method and apparatus for an adaptive equalization technique in a link-hopping system using short transmission bursts for radio communication.
By way of introduction only, the present embodiments provide a method for receiving radio signals in a multiple-link hopping radio system. The method includes hopping among a plurality of radio links to receive short bursts and compensating for amplitude and delay distortion for each radio link. Further, the method includes storing the estimated tap coefficients of an adaptive equalizer and using these tap coefficient values as initial tap weight values for a next received burst on the radio link.
The embodiments further provide a method for receiving radio signals which includes receiving a first burst on a first radio link and determining channel information (i.e., estimating the tap coefficients of the equalizer) about the first radio link using the first burst. The method further includes receiving a next burst such as a second burst of the first radio link using the estimated tap coefficients from the first burst as the initial tap values of the equalizers for the second burst of the first radio link.
The embodiments further provide an adaptive equalization method for use in a multiple-link hopping, burst adaptive modem. The method includes receiving modulated amplitude and delay distorted signals as a series of bursts. The system comprises a fixed fractionally-spaced equalizer configured to equalize a present burst of data using equalizer weights from a previous burst of data generated by an adaptive equalizer per radio link. Further, the method includes estimating and removing the carrier phase offset from the pre-equalized burst.
The embodiments further include an iterative adaptive equalizer unit consisting of two adaptive fractionally-spaced equalizers which equalize the present burst to produce an equalized output signal and provides next burst equalizer weights to a fixed equalizer for equalizing a next burst of data.
The embodiments further provide an adaptive equalization system for use in a multiple-link hopping and burst adaptive modem in steady-state operation. In one embodiment, the adaptive equalization system includes a memory unit and a fixed equalizer for equalizing a present data burst of a present link for which the stored tap coefficients pertains to a previous burst. The adaptive equalizer unit includes a first stage that pre-compensate the amplitude and phase distortion of each radio link using a fixed equalizer in the received burst of data, a carrier phase offset removal stage coupled to the first stage and an iterative adaptive equalizer stage coupled to the carrier phase removal stage. The iterative adaptive equalizer is composed of two fractionally-spaced adaptive equalizers. The first adaptive equalizer is required to provide initial tap estimates for the second adaptive equalizer. The adaptive equalizer system further includes a memory unit for storing the tap coefficient values of the final stage of the adaptive equalizer operated on the current burst of the present radio link. Further, the stored tap coefficients are down loaded into the fixed equalizer of the first stage of the invention to pre-compensate the amplitude and delay variations of the incoming next burst of the present radio link.
The foregoing discussion of the preferred embodiments has been provided only by way of introduction. Nothing in this section should be taken as a limitation on the following claims, which define the scope of the invention.